Photodiode comprising polarizer

ABSTRACT

A photodiode includes a photosensitive area and a polarizing grating located in front of the photosensitive area. The polarizing grating is formed by a plurality of galvanically conducting filaments.

TECHNICAL FIELD

The invention generally relates to photodiodes and, in particularembodiments, to a photodiode comprising a polarizing grating in front ofthe photosensitive area of the photodiode.

BACKGROUND

Conventional photodiodes transform light into an electric current orvoltage at a pn-junction or pin-junction. Depending on the specificpn-junction, the light, which more generally can be considered as anelectromagnetic wave, may be of ultraviolet or infrared or visiblefrequency spectrum. For transforming light of different wavelength aphotodiode may comprise silicon for detecting visible light in the rangeof up to 1 μm or may comprise germanium for detecting light in theinfrared spectrum for up to 1.8 μm or other conventional semiconductingmaterials for transforming light into current or voltage.

When the photosensitive pn-junction is exposed to light, the incidentphotons generate pairs comprising a mobile electron and a correspondingpositively charged hole, thus producing a current flow, as the chargecarriers move into the zones of opposite doping due to the diffusionvoltage. For affecting a pair of an electron and a hole, the photonsnecessarily must yield an energy exceeding the band gap of theparticular photosensitive pn-junction. The current affected by theimpinging photons to a large extent is proportional to the amount ofenergy comprised in the photons until saturation is encountered.

Photodiodes can be used in a variety of circuits, wherein thephotodiodes exhibit different properties. In one embodiment, when thephotodiode is operated without any bias, the current flow out of thediode is limited and a voltage builds up, the diode thus becomingforward biased. This causes a so-called dark-current flowing in anopposite direction to the photo current and which in the end causes thephotovoltaic effect in solar cells. In another embodiment, the so-calledphotoconductive mode, a reverse bias may be applied, which decreases thecapacitance of the pn-junction and reduces the response time of thediode. In still another embodiment a comparatively high reverse bias maybe applied to the diode, which affects a multiplication of each chargecarrier by the avalanche effect, thus affecting an internal gain in thephotodiode.

Photodiodes are used in a variety of applications, for example, inconsumer electronics or any other application wherein light must bedetected. In some particular applications polarizers have been used forpreventing light of a dedicated polarization to pass through thepolarizer and to affect a corresponding current or voltage in thephotodiode. In one embodiment, polarizers have been used in combinationwith photodiodes to differentiate between lights of differingpolarization.

The polarizers used in these conventional photodiodes are discretepolarizers. That is, the polarizers are made of glass or transparentplastics in a standalone production process, which is separate from theproduction process of producing a semiconductor. In other words, theprocess is separate from producing a photodiode as a conventionalsemiconductor using conventional semiconductor production methods. Whenthe polarizers were produced they were placed in front of thephotosensitive material of the photodiode. The production of thesephotodiodes accordingly comprises the production of the polarizer and aphoto diode, both as separate items, and as a micromechanical assemblyof the polarizer in front of the photosensitive area of the diode. Asmicromechanical assemblies are costly and prone to errors, there is aneed for an alternative.

SUMMARY

According to one aspect, a photodiode includes a photosensitive area anda polarizing grating located in front of the photosensitive area. Thepolarizing grating is formed by a plurality of galvanically conductingfilaments.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing is included to provide a further understandingof the present invention and is incorporated in and constitutes a partof this specification. The drawing illustrates embodiments of thepresent invention and, together with the description, serve to explainthe principles of the invention. Other embodiments of the presentinvention and many of the intended advantages of the present inventionwill be readily appreciated, as they become better understood byreference to the following detailed description.

The sole FIGURE depicts a schematic representation of a photodiodecomprising a polarizing grating according to an embodiment of theinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawing, which forms a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or other changes may be made without departingfrom the scope of the present invention.

The sole FIGURE depicts a schematic illustration of a photodiode 100comprising a polarizing grating according to an embodiment of theinvention. Cartesian coordinate system 110 is used in the following fordescribing the photodiode, wherein the directions of x and y areparallel to the paper plane and the direction of z is perpendicular tothe paper plane of the drawing and is directed away from the observer ofthe FIGURE.

In one embodiment, photodiode 100 comprises a layer of a firstsemiconducting material 120 adjoining a layer of a second semiconductingmaterial 130, wherein the layers form a pn-junction. First materiallayer 120 may be a p+ doped semiconductor of a translucent semiconductormaterial, wherein translucent shall describe the property thatsemiconductor 120 is diaphanous for light of the specific wavelengthaccording to the desired characteristics of the diode. In an alternativeembodiment layer 120 essentially may not be of translucent material, butwherein the layer thickness is small and in this way allows light topenetrate the pn-junction.

Light impinging the surface of the diode as indicated by arrow 140 mayaccordingly pass layer 120 and then impinge the pn-junction to cause theappearance of pairs of charge carriers 150, which in turn cause acurrent flow as indicated by the gauge. In this way light, i.e.,photons, impinging on the photosensitive area of the photodiode causes acurrent flow.

In the photodiode, a pair of charge carriers is produced if a photon ofsufficient energy hits the semiconductor material, wherein the photonmay be of any polarization direction. The photosensitive area of thephotodiode accordingly can be characterized as the area where animpinging photon affects a pair of charge carriers contributing to thecurrent.

Photodiode 100 comprises a polarizing grating 160 located in front ofthe photosensitive area, wherein the polarizing grating is formed by aplurality of galvanically conducting filaments. Light 140 accordinglymust pass grating 160 before the photons produce a pair of chargecarriers.

The parallel filaments of grating 160 are directed perpendicular to thetraveling direction of light 140. In the depicted embodiment thefilaments of grating 160 are directed in the direction of the x-axis asindicated by coordinate system 110. Considering that light 140 travelsin the direction y, i.e., particularly in the direction opposite asindicated by direction y, grating 160 is located in front of thephotosensitive area of the photodiode. Accordingly the light is forcedto pass grating 160 before it may produce a pair of charge carriers inthe photodiode.

Since the filaments of the grating are of a conducting material, lightpolarized in the direction of the filaments, i.e., in the direction ofthe x-axis, affects an oscillation of the electrons in the filaments,the oscillating electrons thus reflecting the electromagnetic wave oflight in the direction of the filaments. The grating in this wayprevents light polarized in the direction of the filaments to pass thegrating. Hence, only light polarized in the direction of the z-axis maypass the grating and may hit the photosensitive area of the photodiode.Consequently only light polarized perpendicular to the grating will passthe grating and will cause pairs of charge carriers and a resultingcurrent or voltage in the diode.

In one embodiment, photodiode 100 accordingly can be used to selectivelydetect light polarized in a z-direction. Any light 140, which does notat least comprise a portion polarized parallel to the z-direction willnot affect a pair of charge carriers in the photodiode 100 andaccordingly will not affect any current or voltage in the diode.Accordingly the photodiode 100 comprising the polarizing grating 160 maybe used for detecting light comprising a portion polarized parallel tothe z-direction.

The proposed photodiode 100 accordingly comprises a photosensitive areaand a polarizing grating 160 located in front of the photosensitivearea, wherein the polarizing 160 grating is formed by a plurality ofgalvanically conducting filaments.

The filaments of the grating 160 may be produced from a translucentmaterial. In one embodiment, the filaments may be produced of indium tinoxide (ITO), which is a translucent and conducting material. Since ITOis translucent, it leaves light unaffected if polarized perpendicular tothe direction of the filaments.

In another embodiment, the grating 160 at the same time may serve as anelectrode that can be biased. As mentioned above, a biasing voltage canbe used in a diode to decrease the capacitance of the diode whenoperating the diode with reverse bias, thus reducing the response timeof the diode. The grating accordingly may comprise an interconnection161 for galvanically coupling the filaments of the grating.Interconnection 161 may be coupled to a conductor allowing the grating160 to be coupled to a voltage source 170 for applying a biasing voltageto the diode.

In still another embodiment, not shown in the FIGURE, the diode mayfurther include an intrinsic layer between the p-doped and the n-dopedlayer. The intrinsic layer may be of a weakly doped or undopedsemiconducting material. The PIN photodiode accordingly exhibitsproperties of a conventional PIN diode and a photodiode. The intrinsiclayer affects a reduced capacitance of the junction. Also PINphotodiodes are more stable across temperature and can be produced lesscostly. However, PIN photodiodes are less sensitive to light thanphotodiodes without an intrinsic layer. In this way the photodiode mayfurther comprise an intrinsic layer between its anode and cathode layer,thus forming a PIN photodiode.

In another embodiment, the polarizing grating 160 may be covered byanother layer (not shown) of material, for example, a layer havingspecial properties. The layer may serve as protection against mechanicalimpacts, which may destroy the filaments of the grating 160 or the layermay be non-reflecting, such that incident light is absorbed but notreflected by that layer. In still another embodiment the polarizinggrating 160 may be embedded in any layer located in front of thephotosensitive area of the diode.

The semiconductor layers of the photodiode 120/130 as described may beproduced using conventional production methods followed by applicationof the polarizing grating 160. In one example, the polarizing grating160 may be produced by applying the material as a continuous layer,which is subsequently patterned using conventional methods. Thecontinuous layer may be deposited in one example by chemical vapordeposition (CVD) or electrochemical deposition (ECD) or atomic layerdeposition (ALD). The layer may be patterned using conventional wet ordry etching methods, wherein, in one embodiment, conventionallithographic methods are used. In an alternative embodiment, conductivefilaments may be produced within an existing layer of material byaltering material properties accordingly. In one embodiment, filamentshaped portions of a material layer may be changed to conducting byimplanting ions into these portions, wherein the implantation processmay be a high-energy implantation for injecting the ions deeply into thesubstrate, thus forming conducting filaments below the surface of thelayer at an arbitrary depth.

Since the polarizing grating 160 in this way can be produced usingconventional processing steps, for example, to produce a polarizinggrating 160 from indium titanium oxide the process of forming thegrating may be integrated into the semiconductor production without anymicromechanical assembly.

The described photodiode may be used as discrete assembly part, forexample, in a detector that distinguishes between lights of differentpolarizations. Alternatively, the described diode may be used as part ofan integrated circuit or in a plurality of photodiodes, wherein thephotodiodes may be used to detect lights of different polarizations.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. An apparatus comprising: a photodiode comprising a photosensitivearea; and a polarizing grating disposed over the photosensitive area,wherein the polarizing grating comprises a plurality of conductingfilaments, wherein the filaments of the polarizing grating are coupledby an interconnection, and wherein the polarizing grating is anelectrode configured to bias the photodiode.
 2. The apparatus of claim1, wherein the filaments comprise translucent material.
 3. The apparatusof claim 1, wherein the filaments comprise indium tin oxide.
 4. Theapparatus of claim 1, wherein the photodiode comprises an anode layeradjacent a cathode layer, the anode layer having an oppositeconductivity type of the cathode layer.
 5. The apparatus of claim 4,further comprising an intrinsic layer between the anode layer and thecathode layer.
 6. The apparatus of claim 1, wherein the photodiodecomprises: a p+ layer having a top surface and an opposed bottomsurface; an n+ layer adjacent the bottom surface; and wherein thephotosensitive area comprises the top surface.
 7. The apparatus of claim1, further comprising a non-reflecting layer disposed over thepolarizing grating.
 8. An apparatus comprising: a light sensitive devicecomprising a first semiconductor layer of a first conductivity type; asecond semiconductor layer of a second conductivity type over the firstsemiconductor layer, the second conductivity type opposite the firstconductivity type; and a polarizing grating located over the secondsemiconductor layer, wherein the polarizing grating comprises aplurality of conducting filaments, wherein a bias voltage is applied tothe first semiconductor layer, and wherein the bias voltage is appliedthrough the polarizing grating.
 9. The apparatus of claim 8, furthercomprising a detector coupled between the first semiconductor layer andthe second semiconductor layer.
 10. The apparatus of claim 9, whereinthe detector comprises a current detector.
 11. The apparatus of claim 9,wherein the detector comprises a voltage detector.
 12. The apparatus ofclaim 8, further comprising an intrinsic semiconductor layer between thefirst semiconductor layer and the second semiconductor layer.
 13. Aphotodiode comprising: a photosensitive area; and a polarizing gratinglocated in front of the photosensitive area, wherein the polarizinggrating comprises a plurality of galvanically conducting filaments,wherein the filaments of the polarizing grating are galvanically coupledby an interconnection, and wherein the polarizing grating is anelectrode for biasing the photodiode.
 14. A light sensitive devicecomprising: a first semiconductor layer of a first conductivity type; asecond semiconductor layer of a second conductivity type over the firstsemiconductor layer, the second conductivity type opposite the firstconductivity type; and a polarizing grating located over the secondsemiconductor layer, wherein the polarizing grating comprises aplurality of galvanically conducting filaments, wherein a bias voltageis applied to the first semiconductor layer, and wherein the biasvoltage is applied through the polarizing grating.